In the manufacture of tubing by forming flat metal strips into a hollow tubular form, then welding the seam formed by the abutting longitudinal edges, it is difficult to stop and restart the manufacturing process without creating a hole in the welded seam. For long runs, it is necessary to stop the manufacturing process to resharpen the electrode of the welder or add new stock. This becomes of considerable importance when one or more wires are being fed into the hollow tubing as it is being formed and when the hollow tubing is being continuously filled with a ceramic powder to insulate the one or more wires from each other and from the metal tube sheathing these wires. It is vital that no holes be generated along the seam during a stop and subsequent restart of the process so that the ceramic powder will be contained within the tubing as it is formed and not contaminate the welding area.
The problem of holes being formed in the welded seam upon restarting of the tube forming machine after having been stopped was addressed by Filler in British Patent No. 2 237 223 A. In the process of manufacturing mineral insulated cables disclosed by Filler, the axial position of the welding heads are adjustable to enable a new weld to overlap a portion of the seam welded prior to the process being stopped. Filler provides a servo mechanism which adjusts the position of the welding heads so that the desired weld overlap occurs.
The present invention is an improvement ever the tube forming method and apparatus taught by Filler.